General description of procedure, equipment, technique

Dual-energy subtraction (DES) radiography is a recently developed technique that incorporates both standard, high-energy and low energy (i.e., bone) images as part of a routine chest radiographic examination. DES offers two principal advantages over conventional digital chest radiography. By providing a bone image at low kVp (low energy), calcified or bony thoracic structures are easily delineated, allowing assessment of bony abnormalities that can simulate disease, such as a bone island, costochondral osteophyte, or healing rib fracture, each of which may mimic a solitary pulmonary nodule.

In addition, the bone image can be subtracted from the composite, high kVp (high energy) image, thereby producing a soft-tissue image without impeding bony structures. This process allows for better detection of lung nodules and pneumonia, which, when subtle, can be obscured by overlying ribs and be difficult to detect. Digital chest tomosynthesis is a modern implementation of standard tomography. The technique employs an x-ray tube that moves in an arc relative to the patient and a recording device to produce a series of contiguous frontal (coronal) projections or slices from the anterior to posterior chest. Structures are sharply delineated in successive planes. Only one commercially available unit combines chest tomosynthesis with dual-energy subtraction capabilities for digital chest radiography.

Indications and patient selection

If available, DES radiography is considered the procedure of choice for routine chest imaging. Specific indications for DES chest radiography following standard (non-DES) chest radiography include assessment of focal opacities on standard radiographs thought likely to represent bony or calcified densities. The ability to depict calcified nodules or benign bone abnormalities on the bone image obviates the cost and radiation exposure of a chest CT scan and is particularly useful.

Another indication for DES chest radiography following standard chest radiography is confirmation that a focal opacity seen on standard radiography is an intra-pulmonary lesion that warrants further assessment with a chest CT scan. Focal opacities that raise concern for solitary pulmonary nodules are more easily identified as intra-pulmonary on the soft-tissue image produced by subtracting the bone image from the composite image.

Indications for chest tomosynthesis have yet to be elucidated.

Initial experience suggests that the technique is useful for detecting small pulmonary nodules and is likely to be superior to conventional chest radiography in this regard. However, routine use of chest tomosynthesis involves additional radiation exposure unless frontal tomosynthesis obviates the need for lateral images. Under these circumstances, the exposure dose for tomosynthesis is similar to that for conventional two-view, frontal and lateral chest radiography.

Patients with a suspected intrapulmonary opacity noted on conventional frontal chest radiography may benefit from tomosynthesis to distinguish true lung nodules from bony abnormalities or overlapping structures, as this technique is less expensive than chest CT scanning and is associated with a lower radiation dose. The relative roles of tomosynthesis and DES chest radiography in this regard are unclear. Perhaps the greatest potential utility of chest tomosynthesis lies in the follow-up of nodules previously identified by chest radiography or chest CT, where tomosynthesis offers a lower-dose, lower-cost option than repetitive CT scans.

Contraindications

There are no contraindications to performing DES chest radiography. Since the technology employed for this examination is in the form of dedicated fixed units in radiology departments, only patients who are capable of undergoing upright chest radiography (typically, outpatients and mobile inpatients) can be imaged using this technique.

The radiation exposure necessary to produce a good quality, subtracted frontal image is higher than that needed for conventional frontal chest radiography. Consequently, the radiation dose for two-view DES chest radiography is approximately 15 percent greater than that for standard imaging. In order to reduce radiation exposure using the DES technique to that of conventional two-view digital chest radiography, some operators reduce the radiation dose for the lateral view while increasing it for the DES frontal projection.

Chest tomosynthesis is also associated with a higher effective radiation dose compared with conventional chest radiography. The average effective dose for a two-view chest radiographic examination is 0.06 milliSieverts (mSv), while the dose for chest tomosynthesis is 0.12 mSv. The effective dose of a standard chest CT examination is approximately 4-5 mSv.

Details of how the procedure is performed

Dual-energy chest radiography is performed using a dedicated unit capable of producing two frontal images (see below). Since a DES lateral view carries significantly higher radiation exposure than does the frontal projection and is not be useful for nodule detection, the technique is limited to the frontal projection.

Two commercial DES radiographic units are in use, each of which uses a different technique to produce the two frontal images. One unit generates two sequential exposures, one at 60 kVp (the bone image), and the other at 120 kVp (the composite or standard image); the images are generated within a 150-millisecond interval. Another unit is based on a single exposure of two storage phosphor plates separated by a copper filter. The first plate records the standard image, and the second records a high-energy image as the first plate and filter absorb the lower energy photons.

Regardless of the technique used, ultimately, three frontal images are available for viewing--two generated primarily, as described above, and one derived electronically. In this process, a composite, standard frontal image is derived from the high kVp exposure analogous to a conventional digital chest radiograph (Figure 1). A bone image (Figure 2) is produced from either a second frontal exposure at 80 kVp, that takes advantage of the bone's or calcium's higher absorption of the lower energy photons, or from subtraction of the standard and high kVP images produced from the single-exposure, filtering technique. Finally, an electronically derived soft-tissue image is obtained by subtracting the bone image (Figure 3).

Figure 1.

Normal frontal digital chest radiograph.

Figure 2.

Bone image from dual energy chest radiography.

Figure 3.

Soft tissue image from dual-energy chest radiography.

The lateral radiographic image obtained using a DES unit is identical to that generated using a conventional unit (Figure 4). A lower dose is often employed for the lateral view in order to compensate for the increased radiation exposure needed for the dual energy frontal view (see above).

Figure 4.

Lateral digital chest radiograph.

Chest tomosynthesis is performed using a single ten-second exposure, which is well within the breath-holding capabilities of most ambulatory patients. Sixty contiguous, 4 mm-thick coronal tomographic images are generated. The x-ray tube courses over a 35-degree vertical arc while the patient and the detector remain stationary.

Interpretation of results

The images routinely obtained using DES chest radiography are interpreted on a picture archiving and communication system (PACS workstation and disseminated electronically for system-wide access as with any digital radiographic study. The bone and soft-tissue images serve as adjuncts to the composite image. The bone image allows calcified structures, such as calcified granulomas (Figure 5, Figure 6) or hamartomas, bony abnormalities, calcified vessels, and myocardial and pericardial calcifications, to be readily identified. The soft-tissue image allows for an unimpeded view of the lungs by "subtracting out" the ribs, thereby improving detection of focal pulmonary abnormalities, particularly those obscured by the ribs on the composite view.

Figure 5.

Frontal chest radiograph demonstrates a small retrocardiac nodule.

Figure 6.

The subtracted bone image shows the nodule to be calcified, therefore representing a granuloma.

Chest tomosynthesis images are also viewed on a PACS workstation. The radiologist can "cine" through a stack of images, comparing the tomographic slices to the conventional frontal radiograph (Figure 7, Figure 8).

Figure 7.

Frontal chest radiograph shows a subtle lingular nodule.

Figure 8.

Tomogram through mid chest more clearly delineates the lesion.

Performance characteristics of the procedure (applies only to diagnostic procedures)

DES chest radiography has obvious utility in detecting calcification within benign lung nodules and in distinguishing bony abnormalities from true intrapulmonary lesions. Several published studies that have evaluated the technique have demonstrated improved detection of non-calcified pulmonary nodules--specifically, small lung cancers--over that with conventional chest radiographic methods. DES chest radiography appears to be particularly useful when pulmonary lesions either partly or completely overly a rib (95 percent of lung cancers) and are initially missed radiographically then visible in retrospect.

In a recent large, multi-institutional evaluation on the utility of chest tomosynthesis in the detection and management of lung nodules, with CT as the gold standard, tomosynthesis was superior to conventional radiography and DES in detecting nodules.

Outcomes (applies only to therapeutic procedures)

N/A

Alternative and/or additional procedures to consider

Where available, DES chest radiography should be employed as the standard imaging technique for clinically indicated chest radiography. When used as an additional diagnostic tool in evaluating a possible lung nodule, or when routine DES chest radiography detects a probable intrapulmonary nodule, either chest tomosynthesis or a CT scan can be employed for more definitive localization and characterization.

When chest tomosynthesis is used as a diagnostic tool in a patient with a suspicious focal density noted on conventional chest radiography, a CT scan can be used for confirmation, localization, and characterization of the density. DES chest radiography is an alternative technique in this setting, particularly if the finding on conventional chest radiography might represent a bony or calcified density. In a patient with an intrapulmonary lesion seen on chest tomosynthesis, thin-section CT scanning is necessary for further evaluation.